Atmospheric carbon dioxide (CO2) is increasing and having an important effect on the regulation of the earth's temperature. Furthermore, roughly 30% of anthropogenic CO2 leaves the atmosphere and enters the earth's oceans and other large bodies of water. These water bodies typically act as large sinks of CO2, via dissolution of CO2 as carbonic acid, with concomitant changes in ocean pH. Unfortunately, devices for directly measuring pH in the natural environment are unreliable when deployed for any length of time. In coastal systems where changes in salinity are common and biofouling extensive, measuring pH can burdensome. Alternatively, measurements of changes in the partial pressure of CO2 in the ocean can provide valuable and reliable information about changes in the acidity of the ocean. Nearshore coastal water pH measurements can also be made providing similar information.
Methods for measuring pCO2 in oceans have mainly focused on measuring acidification in open ocean settings. These methods assume that acidification is driven by a stable air-sea CO2 equilibrium, such that measurement of the ocean's pCO2 is reflective of atmospheric pCO2. The technology depends on large, expensive, and sparse autonomous buoys to characterize hundreds to thousands of km2 of ocean surrounding them. Buoy data are supplemented by data from large, expensive, and sparse oceanographic research vessel transits.
Due to the complexity of nearshore coastal waters, an air-sea equilibrium rarely occurs and measurements must be made at much higher frequencies over space and time. Increased frequencies can assist to reliably characterize pCO2 and pH. In nearshore waters the carbon cycle is much more complicated than the open ocean, and land-sea interactions are frequently more acute than air-sea interactions. Nearshore waters are further complicated by biological activities such as photosynthesis and respiration and the pCO2 of water is far more dynamic than in the open ocean. Changes in pCO2 are more rapid than in open ocean waters and pCO2 can vary significantly over very short distances and time spans. Measurements must therefore be made much more frequently and much more densely in order to capture the natural temporal and spatial variability present. Thus, use of cheaper, more numerous, portable, and easily deployable pCO2 instruments is desirable.